(1) Field of the Invention
The present invention relates to an in-mold decorated illumination panel, and more particularly to an in-mold decorated illumination panel having an indicator portion printed on a back surface of an indicator sheet.
(2) Description of the Related Art
It is known that an in-mold decorated illumination panel is used for a dash board of an automobile, for example. Such an in-mold decorated illumination panel is constructed of an indicator sheet having an indicator sheet such as an indicator symbol printed on a surface of the indicator sheet and a molded layer integrally formed on a back surface of the indicator sheet by injection molding. The indicator portion is illuminated from the side of the molded layer to effect indication of the indicator portion on a front surface of the indicator sheet. The indicator portion is printed on the front surface or the back surface of the indicator sheet.
In the former type such that the indicator portion is printed on the front surface of the indicator sheet, minute adjustment of illumination by ink cannot be effected because of unevenness of the front surface of the indicator sheet. Further, as the indicator portion is printed on the front surface of the indicator sheet, a crimp pattern of the front surface of the indicator sheet cannot be effectively utilized.
In order to effect the minute adjustment of illumination by adjusting a thickness and color of an ink printed layer and effectively utilize the crimp pattern of the indicator sheet, the latter type such that the indicator portion is printed on the back surface of the indicator sheet is currently adopted. FIG. 7 shows a construction of a conventional in-mold decorated illumination panel 1 of the latter type, in which an ink printed layer 3 including an indicator symbol (not shown) is formed on a back surface of an indicator sheet 2, and a molded layer 5 formed from ABS resin in directly formed on the ink printed layer 3 by injection molding.
However, the conventional in-mold decorated illumination panel 1 has a shortcoming such that the ink of the printed indicator symbol is dissolved away in the manufacturing step of injection molding. This problem is not so remarkable in the case that the panel 1 is relatively small in size as in the application to knobs, for example, but it becomes a serious problem in the case that the size of the panel 1 is large. That is, the larger the size of the panel 1, the more an injection quantity of hot molten resin upon injecting molding. As a result, a large injection pressure of the molten resin is directly applied to the ink printed layer 3, so that a large amount of ink of the indicator symbol is dissolved away. FIG. 8 illustrates the problem of dissolution of the ink. As shown in FIG. 8, the ink of a printed indicator symbol 6 is dissolved away to result in formation of a defective ink line 7 along an inside edge 5a of the molded layer 5 by the pressure of the molten resin upon injection molding. The formation of such a defective ink line 7 causes a problem such that illuminated indication of the indicator symbol 6 on the front surface of the indicator sheet 2 becomes unclear. Accordingly, in the case that the in-mold decorated illumination panel 1 is large to some extent, it is necessary to adopt a method of attachment of a face plate or multi-color molding, which causes complication of a manufacturing step of the panel 1.
Further, while there occurs warping of an illumination panel upon molding in general, it is known that the warping of the panel is physically corrected by providing a rib for preventing the occurrence of warping upon molding. However, in the in-mold decorated illumination panel 1 having the indicator sheet 2 and the molded layer 5 fixedly attached to each other, a shrinkage sheet 2 is different from that of the molded layer 5. It is therefore impossible to correct the warping of the panel 1 by providing the above-mentioned rib. Thus, the warping of the panel 1 as shown in FIG. 9 cannot be avoided.